It is widely accepted that disequilibrium-doubt and cognitive conflict- plays a crucial role in promoting and directing cognitive development. Unfortunately, however, the study of cognitive development has relied on procedures that tend to mask individuals' uncertainty. Standard testing procedures encourage closure on single answers regardless of whether, from the point of view of the individual, more than one alternative appears possible and the endorsement of any one alternative cannot be made with complete confidence. Attempts to detect subjects' awareness of uncertainty through modifications of standard procedures have not been very successful, particularly when it comes to unveiling judgment uncertainty among preadolescent children. Recent studies have indicated however, that the adoption of completely different testing methods can unveil judgment uncertainty when it is present. Application of these procedures suggest that judgment uncertainty is much more prevalent than previously assumed. It is the author's opinion that given the likely importance of uncertainty as a dynamic force in cognitive development, we need to begin a reinvestigation of cognition using these new methods with an eye to charting the course of judgment certainty over the course of concept development. Using conservation as a prototype concept, the proposed research will use one of these new methods to explore how judgment certainty changes over time as a function of age, experience and training.